The long term aim of our research is to elucidate the mechanism of visual transduction in rods. It is generally believed that transduction is mediated by an intracellular messenger, which serves as a link between the bleaching of visual pigment and the rod's electrical response to light. A large body of indirect evidence suggests that ionized calcium is the messenger in rods. However, a definitive proof of this hypothesis remains elusive due to a lack of experimental methods for studying the light regulation of intracellular calcium in rods. Therefore, we propose intracellular and extracellular calcium measurements in intact cells to rigorously test this hypothesis. We have developed a novel method, utilizing ion selective microelectrodes in combination with patch clamp techniques, to measure intracellular calcium in rods. Intracellular measurements will allow, for the first time, a comparison between the light regulation of calcium and the expected behavior of the intracellular messenger. The intracellular measurements are totally without precedent, and are the first experimental approach capable of demonstrating whether or not calcium is the internal messenger.